This application is related to U.S. patent application Ser. No. 932,744, filed Aug. 10, 1978.
Plastics are finding wider and wider areas of application due to their light-weight, high-strength characteristics. With these advantages plastics bring certain problems. More specifically, due to the tendency of plastic materials to cold flow under load, clamping and fastening the plastic members by conventional methods becomes impossible. This is because standard screws and bolts depend on pressure from the joint to prevent fastener back off and loosening of the joint. When the plastic cold flows, it relieves the joint pressure leaving the fastener free to vibrate out of the joint.
A further problem arises when these plastic members are fastened to a relatively thin sheet metal support. Because of the thinness of the material, there are only a few threads of the fastener engaged with the workpiece. Hence the stripping torque (that is, the amount of rotational force necessary to ream the threads out of the workpiece) is relatively low. The higher the stripping torque, the greater the resistance to fastener back off. Thus the problem of fastening plastic materials becomes most accute when fastening them to thin sheet metal.
The thinness of the metal forming the support creates a third related problem. The drive torque (or amount of rotation force necessary to insert the fastener) will vary depending on such parameters as the local hardness of the sheet metal, the hardness of the fastener threads and the angle of fastener insertion, among others. The workman would like to select a torque for the power driver which (1) will insure every fastener will be driven (2) without over-torquing or stripping any. Due to the low stripping torque in the thin metal, the workman has a very narrow range of torques, if any, which will permit both requirements to be satisfied.
It is an object of the present invention to provide a fastener unit capable of fastening plastic to thin sheet metal which, in operation, will have a greatly increased stripping torque inspite of the characteristics of the plastic and the sheet metal. This will enhance the capabilities of the unit as a fastener and facilitate its use by enlarging the range of torques with which it may satisfactorily be driven.
These and other objects of the invention are accomplished by a self-tapping fastener which is assembled with a spacer to form a unit. The spacer is a cylindrical collar which has a predetermined length corresponding to the thickness of the plastic material and an inside diameter which exceeds the thread crest diameter of the fastener by approximately twice the thickness of the sheet metal workpiece. The spacer permits the upper surface of the plastic to be lightly clamped by a material-engaging means which may be on the lower side of the head of the fastener or a flange on the spacer. A portion of the edge region of the hole in the workpiece which surrounds the shank of the fastener will be extruded into the gap between the spacer and the thread crests greatly increasing the stripping torque.